JOHANNESBURG, May 10 -- Zimbabwean opposition leader Morgan Tsvangirai announced Saturday that he will soon return to his country to participate in a presidential runoff election despite a surge in political violence against his supporters.

Tsvangirai called on southern African regional leaders to ensure that the campaign for the runoff, which has not yet been scheduled, is free of violence and is overseen by a reformed electoral commission and international observers. He stopped short of saying that his participation depended on those conditions.

"We will contest the runoff, and the people will finally prevail," Tsvangirai said at a news conference in Pretoria, the capital of South Africa, where he has been based for the past month. "The people have spoken before, and the people will speak again. I am ready, and the people are ready for the final round."

The announcement reversed weeks of vows by the opposition Movement for Democratic Change to boycott any new election against President Robert Mugabe. The party has maintained that Tsvangirai narrowly received an outright majority in the March 29 election and that a second round of voting was therefore unneeded.

But according to the electoral commission's official results, Tsvangirai won the first round with 47.9 percent to 43.2 percent for Mugabe, leaving both short of the majority necessary to avoid a runoff.

The opposition has long complained that the electoral commission, which delayed the release of its results for more than a month, is biased toward Mugabe and inflated his vote count.

"Those MDC issues do not concern our party," said Didymus Mutasa, Mugabe's security minister. "But if they are serious this time around, they will be shocked, because we will proceed without them. We as a party have always been clear that we will go for a runoff."

Saturday's announcement followed weeks of furious internal debate among opposition leaders about how to proceed against Mugabe. But with little sign that he intended to relinquish power after 28 years, Tsvangirai faced few options for continuing his battle against a government that has steered once-prosperous Zimbabwe into such steep economic decline.

Tsvangirai told reporters that he intends to return to Zimbabwe in the next two days. He left the country soon after the election and has publicly expressed concern that he could face arrest or physical assault in Zimbabwe, where he has been repeatedly arrested and severely beaten in the past.

His time away has coincided with a sharply rising campaign of retribution against opposition activists, especially in rural areas that Mugabe's party has long controlled. Ruling party youths have rampaged through towns, dragging Tsvangirai supporters from their homes, then beating and torturing them.

The Zimbabwe Association of Doctors for Human Rights said hospitals are running out of basic medical supplies to treat the injured. In a statement released Thursday, the group put the confirmed death toll from the political violence at 22 but said that "at least double that number have been reported." Other human rights groups have said tens of thousands have been displaced.

Political debate within Zimbabwe has focused on whether the surge in violence has turned more voters away from Mugabe or frightened off Tsvangirai's supporters.

Analyst Eldred Masunungure, speaking from Harare, the capital, said he does not believe that the opposition could win a runoff under current conditions. He said support in the countryside has deteriorated, most markedly in Mashonaland, Mugabe's former political heartland where Tsvangirai won many towns. "The greater part of Mashonaland, now it's drenched in blood," Masunungure said.

Tsvangirai acknowledged the violence against his supporters but said he needs to participate in the runoff to complete his challenge against Mugabe's rule. "Some might say this term 'victory' is cold and callous, given the hardships endured by the people," Tsvangirai said. "We must free ourselves from those who would steal victory from fellow brothers and sisters by using guns, sticks and screwdrivers."